The Black Guillemots of Cooper Island
Identification
1.	Description
This regional feature examines how the number of breeding pairs of a colony of black guillemots
(Cepphus grylle mandtii) has changed over time in northern Alaska. Tracking the population of breeding
adults of this colony through annual studies on the island is key to understanding the implications of
temperature and sea ice changes in this coastal marine ecosystem. Recent major declines in summer
Arctic sea ice extent can affect the distributions and life histories, such as patterns of survival and
reproduction, of Arctic marine species adapted to living near sea ice (Divoky et al., 2015). For example,
the effects of ice reductions and associated increasing ocean sea surface temperatures could impact the
abundance of forage fish in the Arctic, such as the Arctic cod (Boreogadus saida), which is the primary
prey for the region's upper trophic level marine predators. Specifically, black guillemots are ice-obligate
sea diving birds that depend on the local availability of sea ice for their method in hunting prey, as well
as on the prey itself.
Since the 1970s, certain environmental factors have been associated with changing suitability for black
guillemots on Cooper Island, including earlier local spring ice-out and earlier laying of the first egg (Cox
et al., 2017). Overall, when looking at record-setting monthly measurements, the entire Arctic region
has seen consistent declines in sea ice extent, with only new low extent records measured since 1986
(Parkinson and DiGirolamo, 2016). Of particular note, the distance from Alaska's northern coast to the
edge of the summer ice pack began to increase noticeably during the 1990s (Parkinson and DiGirolamo,
2016). The numbers of breeding pairs of this colony are important as an indicator of climate change
because they reflect both physical changes in sea ice and associated implications for the marine
ecosystem species of the region.
This feature consists of the following data series:
• The number of breeding pairs in the black guillemot colony that inhabit Cooper Island along the
north coast of Alaska, starting in 1975 (Figure 1).
2.	Revision History
April 2021: Published feature.
Data Sources
3. Data Sources
This data set was compiled and used for an analysis that appeared as part of a larger assessment of the
Cooper Island area that was published in a journal article (Divoky et al., 2015).
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A collaboration of scientists helped form the Cooper Island Black Guillemot study, which has since
become part of the recently initiated project, Sentinels of the Sea Ice (SENSEI). Comprising 13 teams of
researchers from six countries, the project aims to assess recent and ongoing responses of ice-
associated seabirds and seals to changes in Arctic and Antarctic sea ice.
4. Data Availability
EPA obtained these data in spreadsheet format directly from Dr. George Divoky of Friends of Cooper
Island. The data set is publicly available upon request at: http://cooperisland.org.
Methodology	
5.	Data Collection
This feature provides information on the number of breeding pairs of black guillemot at a specific
location in northern Alaska. A black guillemot colony has been studied annually since 1975 at Cooper
Island, a sand and gravel bar 35 kilometers southeast of Point Barrow, Alaska. Guillemots are cavity-
nesting alcids (an alcid is a bird of the family Alcidae, which includes the murres, guillemots, auklets,
puffins, and murrelets), typically breeding in cavities in scree and talus on rocky shorelines (Cairns, 1980;
Harris and Birkhead, 1985), but all nests used on Cooper Island are in human-made cavities, thereby
giving investigators access to all nests. Creation of nest sites by investigators from 1972 to 1984 allowed
the colony to grow from 10 breeding pairs in 1972 to 220 in 1988 (Divoky et al., 2015). Breeding pairs
exceeded sites in some years as some sites housed more than one pair during the period of maximum
occupancy. Thus, the colony peaked at 220 pairs in 1988. In 2011, all the original sites were replaced
with bear-proof plastic nest cases. The nest cases were provided before the start of egg laying and were
subsequently adopted by the birds.
The number of breeding pairs is determined by counting the number of active nests (nests with eggs).
Starting in 1980, black guillemot adults were also identified through banding during the breeding
season. Birds were banded with a metal band and each received a unique combination of three plastic
color bands allowing for identification of individuals with binoculars. However, because all nests are in
human-made nest sites, it is possible to examine all places where guillemots would be breeding and
determine the number of breeding pairs.
6.	Derivation
Researchers used direct counts of the number of nest sites where egg laying and subsequent incubation
occurred to determine the number of breeding pairs for the colony. This count of breeding pairs
occurred at the peak of the breeding season each year (June, July, and August).
The count of nest sites reflects the number of nests created by investigators that were available each
year, including the original nests on the island when the colony was first discovered.
Scientists observed 85 breeding pairs on Cooper Island in the summer of 2019. Of the 128 chicks that
hatched, 32 fledged. This fledgling success is one-third the rate necessary to maintain the colony,
according to the lead author of this ongoing study, Dr. George Divoky.
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Although no causal link has been established, the precipitous decline in the breeding health of the
Cooper Island colony and the Herschel Islands colony (see Section 8) coincides with extreme conditions
in the wintering habitat for black guillemots. During the winter of 2017-2018, the Bering Sea
experienced record low sea ice conditions from early December through April (Divoky et al., 2018).
To supplement the observations shown in this feature, Figure TD-1 shows a similar analysis from
another long-studied black guillemot colony on Herschel Island, Canada. This additional analysis offers
corroborating context for the overall breeding health of the black guillemot population in the western
Arctic region (Kuyt et al., 1976).
Figure TD-1. Black Guillemot Breeding Pairs on Herschel Island, Canada, 1984-2018
40
35
30
20
10
5
0
1980
1985
1990
1995
2000
Year
2005
2010
2015
2020
Data source: Eckert, 2018
7. Quality Assurance and Quality Control
Quality assurance and quality control involved review by several scientists involved in the annual studies
and familiar with this colony of birds.
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Analysis
8.	Comparability Over Time and Space
Annual studies over the 40+ years since the colony was first discovered have used consistent census
methods for identifying and counting breeding pairs of the birds. The number of breeding pairs is simply
the number of active nests (nests with eggs). As all nests are in human-made nest sites, it is possible to
consistently examine all places where guillemots would be breeding and determine the number of
breeding pairs. In 2011, all of the original sites were replaced with bear-proof plastic nest cases. The
nest cases were provided before the start of egg laying and were adopted by the birds. No known
breeding pairs have used natural nest sites. Beyond nesting cavity installations and nest cases, no
further human interventions have influenced the birds during the breeding season. The observations
have been consistently collected during summer months (June-August).
Cooper Island had more breeding pairs than nest sites in the late 1980s, when some of the larger nesting
sites were occupied by more than one pair. Many additional birds were present but unable to breed due
to the limited number of nest cavities. In some years, the colony had 200+ nonbreeding birds in addition
to about 400 breeding birds. Most of the nonbreeding birds were banded, and many died without
entering the breeding population.
9.	Data Limitations
Factors that may impact the confidence, application, or conclusions drawn from this feature are as
follows:
1.	Nests used on Cooper Island are in human-made cavities, thereby giving researchers access to
all nest contents for study. The creation of nest sites by the researchers from 1972 to 1984
allowed the colony to grow from 10 breeding pairs in 1972 to 200 in 1984; therefore, it
influences the numbers of breeding pairs in the early part of the record.
2.	Changes in the timing and duration of the breeding season significantly influence the likelihood
of successful reproduction. Successful reproduction can also be influenced by competition from
other bird species, the activity of predators such as polar bears, and the success of and
immigration from other black guillemot colonies in the Arctic. The timing and duration of the
breeding season can also change in response to changing temperatures and snow and ice
conditions.
10.	Sources of Uncertainty
While visual field observations and counts of bird species have inherent uncertainties, the Cooper Island
colony—with nesting opportunities only in human-made sites—allows an accurate census of the
number of breeding pairs annually. Uncertainties in this data set are associated with bird tagging errors
(e.g., lost tags) or count recording error. These uncertainties have been identified and minimized,
though not explicitly quantified. More than 95 percent of the guillemots that survive the nonbreeding
season return to the same nest sites, so there is little or no loss of established breeders to other
colonies. Additionally, with the exception of a few individually breeding pairs, the closest guillemot
colonies are more than 400 kilometers away. This means that the birds present on the island are those
that inhabit the island; they are not visitors from nesting colonies elsewhere in the region. For
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reference, the two closest guillemot colonies are at Cape Lisburne in Alaska and Herschel Island in
Canada.
11.	Sources of Variability
While the availability of suitable habitat and prey are critical to thriving guillemot populations, many
factors could contribute to the variability of the number of breeding pairs. Shifts in range, overwinter
survival, temperature and snow cover conditions, the availability of more preferable nesting sites, and
species gender balance are some of the potential sources of variability. In addition, as the count of
breeding pairs depends on the observation of successful egg laying, changes in fertility or nest predation
could influence annual totals.
Immigration from other colonies in the region has contributed to colony growth and also maintenance.
As all birds fledging from the colony are banded, the annual number of immigrants per year has been
known since 1978. A decrease in immigration rates to the island had contributed to the increasing
number of vacant nest sites. The decrease in immigration may be due to a decrease in productivity at
source colonies or to Cooper Island being an undesirable location for recruitment for nonbreeding
prospecting birds.
12.	Statistical/Trend Analysis
This feature does not report on the slope or average rate of change in the figure, nor does it calculate
the statistical significance of these trends or provide confidence bounds. This is primarily due to the
limited, discrete temporal nature of these data.
References
Cairns, D. 1980. Nesting density, habitat structure, and human disturbance as factors in black guillemot
reproduction. Wilson Bull. 92(3):352-361.
https://sora.unm.edu/sites/default/files/iournals/wilson/v092n03/p0352-p0361.pdf.
Cox, C.J., R.S. Stone, D.C. Douglas, D.M. Stanitski, G.J. Divoky, G.S. Dutton, C. Sweeney, J.C. George, and
D.U. Longenecker. 2017. Drivers and environmental responses to the changing annual snow cycle of
northern Alaska. Bull. Amer. Meteor. Soc. 98(12):2559-2577.
https://iournals.ametsoc.Org/doi/10.1175/BAMS-D-16-0201.l. doi:10.1175/BAMS-D-16-0201.1.
Divoky, J.G., D. Douglas, and C. Barbraud. 2018. Survival and breeding response of a sea-ice obligate
seabird following the unprecedented low extent of winter ice in the Bering Sea. American Geophysical
Union Fall Meeting. Washington, DC. Poster: OS43E-2142.
Divoky, J.G., M.P. Lukas, and M.L. Druckenmiller. 2015. Effects of recent decreases in arctic sea ice on an
ice-associated marine bird. Prog. Oceanogr. 136:151-161.
Eckert, C.D. 2018. Black guillemot population monitoring at Herschel Island-Qikiqtaruk Territorial Park,
Yukon: Outcome of the 2018 nesting season. Yukon Department of Environment.
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Harris, M.P., and T.R. Birkhead. 1985. Breeding ecology of the Atlantic Alcidae. In: D.N. Nettleship, T.
Birkhead, and J. Bedard. The Atlantic Alcidae. London, United Kingdom: Academic Press.
Kuyt, E., B.E. Johnson, P.S. Taylor, and T.W. Barry. 1976. Black guillemots' breeding range extended into
the western Canadian Arctic. Can. Field-Nat. 90:75-76.
Parkinson, C.L., and N.E. DiGirolamo. 2016. New visualizations highlight new information on the
contrasting Arctic and Antarctic sea-ice trends since the late 1970s. Remote Sens. Environ. 183:198-204.
doi:10.1016/j.rse.2016.05.020.
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